This application makes reference to, incorporates the same herein, and claims all benefits accruing under 35 U.S.C. xc2xa7119 from an application entitled METHOD OF PRIORITIZING HANDOFF REQUESTS IN MOBILE COMMUNICATION SYSTEM earlier filed in the Korean Industrial Property Office on Jul. 31, 1998, and there duly assigned Serial No. 98-31224.
1. Field of the Invention
The present invention relates to a method of assigning a channel for a handoff in a mobile communication system, and more particularly, to a method of prioritizing handoff requests in a mobile communication system in order to assign free channels to queued handoff requests based on their priorities.
2. Description of the Related Art
In a mobile communication system, handoff is the procedure by which a call is switched to a new communication channel while a user is talking. Communication channel switching may be implemented within a particular cell at one base station or between different cells. During an inter-cell handoff, how to assign the limited number of available channels in each cell is a very significant consideration. The channels can be assigned by prioritizing handoff calls. More specifically, when there are no available channels in a base station associated with a corresponding cell, mobile handoff requests are prioritized and queued. Then, if there is a free channel, the free channel is assigned to the handoff call with the highest priority. A typical channel assignment procedure for handoff in a mobile communication system is depicted in FIGS. 1 and 2.
FIG. 1 is a block diagram of a base station 100 in a cell, which receives channel assignment requests from a plurality of mobile stations 105a, 105b, and 105c located within a handoff region.
Each mobile station 105a, 105b, and 105c sends an RF (Radio Frequency) signal, requesting a new channel assignment. Included in the RF signal is data indicative of the power level detected by the mobile station. Each mobile station 105a, 105b, and 105c samples the transmitted power level on a periodic basis and sends them to the base station 100. The base station 100 receives the sampled power level readings through an antenna 130 and computes the rate of change of power level for the corresponding mobile station.
A demodulator 135 converts the received RF signal to digital data. The digital data includes voice and control data. A processor 140 receives the digital data and extracts the power level data from the voice and control data. The power level data is stored in a memory 145. The voice data is transmitted to a mobile telephone switching office (MTSO) 150 which determines the destination of the data. The processor 140 retrieves the power level data for a particular mobile station and determines from it the cutoff time at which the mobile station will leave the handoff region 120. Cutoff times are used to determine handoff priorities. For example, mobile stations having smaller cutoff times are assigned higher priority than mobile stations having larger cutoff times. Available channels are assigned to handoff requests based on their priorities within a queue in which the handoff requests are arranged according to their cutoff times. These assignments are communicated to the mobile stations using a modulator 170 and an antenna 165.
FIG. 2 is a flowchart depicting a general handoff call process in a base station upon channel assignment requests from mobile stations. Referring to FIGS. 1 and 2, the foregoing will be described in more detail.
Upon receipt of a channel assignment request from a mobile station (step 201), the processor 140 of the base station 100 then determines whether the call is a new call or a handoff call (step 203). If the call is a new call, the base station 100 determines whether any of its channels are free (step 205). If a free channel exists, the mobile station is assigned the channel (step 209). If no channels are free, the processor 140 rejects the call (step 207). If a handoff is requested (step 203), the base station 100 again determines whether any of its channels are free (step 211). If a channel is free, the mobile station is assigned the channel (step 209). If no channels are available, the handoff request is placed in a queue (step 213). If multiple handoff requests exist (step 215), they are prioritized (step 217). A determination is made whether any of the mobile stations requesting a handoff have left the handoff region 20 of FIG. 1 (step 219). If the mobile station has left the handoff region 120 and has not been assigned a new channel by the base station 100, the call is aborted (step 221). Once a channel is available (step 223), the channel is assigned to the queued handoff request having the highest priority (step 225).
However, since the movement of the mobile stations requesting a handoff cannot be accurately predicted, it is also impossible to exactly determine which handoff call will be first cut off. Yet the determination can be made with a relatively high accuracy by estimating the cutoff time of each call. The power level of the currently serving base station measured at the mobile station and the rate of change of the power level is used to make such an estimation. The power level is a significant factor which determines a cell boundary and defines a handoff region. From here the term xe2x80x98cutoff timexe2x80x99 is used to refer to the estimated cutoff time.
FIG. 3 is a flowchart depicting a conventional method of prioritizing handoff requests in step 217 of FIG. 2.
Referring to FIG. 3, the minimum cutoff time MINt is set to an initial value which is typically infinity (step 301). Next, the base station 100 selects one of the queued handoff requests for which the cutoff time will be determined (step 303). The cutoff time for a particular mobile station i is determined by
T(i)=(Pminxe2x88x92Pl(i))/Rxe2x80x83xe2x80x83(1)
where T(i) is the cutoff time for a mobile station i; Pmin is the cutoff threshold power, Pl(i) is the current power level detected by the mobile station i and transmitted to a base station, and R is the rate of change of power level for the mobile station i, given by
dPl(i)/dtxe2x80x83xe2x80x83(2)
The processor 140 retrieves the cutoff threshold power Pmin, the current power level Pl(i), and the rate of change of power level R for the selected handoff request from the memory 145 (step 305). Then, the processor 140 calculates the cutoff time T(i) using Eq.1 (step 307). The processor 140 compares T(i) with MINt (step 309). If T(i) less than MINt, MINt is set to T(i) (step 311). Otherwise, it is determined whether there are any other queued handoff requests (step 313). If there are additional queued handoff requests, steps 303-311 are repeated. If there are no additional queued handoff requests, the processor 140 sets the handoff call of the mobile station having the most critical state to the highest priority (step 315). This corresponds to the mobile station i with T(i)=MINt. This means that the highest priority is given to a handoff call having the smallest cutoff time among queued handoff requests and when a channel becomes available, the mobile station is assigned the channel.
The conventional method of prioritizing handoff requests is based on the assumption that all handoff calls need channels of the same capacity. Yet the channel capacities may vary in actual situations. For example, in areas where radio multimedia service is supported, it may be more appropriate to change the priority of a handoff according to service type. Therefore, there is a need for a new channel assignment scheme in which required channel capacity and service type are taken into account.
An object of the present invention is to provide a method of prioritizing handoff requests in a mobile communication system supporting a multimedia service.
According to an embodiment of the present invention, when there are a plurality of queued handoff requests, the handoff requests are prioritized based on the channel capacity, service type, and cutoff time of the call, and the next free channel is assigned to the handoff request with the highest priority.